The melt overflow process typically includes a molten material that is contained within a hearth which is a refractory lined furnace which has four sides, one of which has at least a portion with a top edge which is lower than the upper surface of the molten material. A rotating, heat extracting substrate, such as a water cooled drum, is placed in very close proximity to the hearth in the region where the one wall is lower than the surface of the molten material. The drum is rotated and the molten material cools and freezes against the surface of the drum, and the solidified material adheres to the drum and "rides" the drum upward and over the top of the drum. In this process fibers or a continuous strip of solidified material is formed directly from molten material.
One problem that exists with this melt overflow process is the difficulty in emptying all of the molten material out of the hearth. It is also difficult to maintain a constant circumferential height of molten material in contact with the rotating drum throughout the overflow process. It is necessary to keep the height of material in contact with the drum constant in order to maintain a uniform thickness in the finished strip or fiber.
In U.S. Pat. No. 4,907,641, Gaspar pivotally attaches a hearth to pivot about the axis of the conventional rotating drum. The hearth, filled with molten material, is then rotated about the axis of the drum and slowly poured onto the drum, overflowing over one edge of the hearth. The edge of the hearth over which the liquid flows maintains a constant radial distance from the outer surface of the drum, thereby avoiding both contact with, and possible damage to, the drum and excessive spacing from the drum.
That apparatus has disadvantages which include the inability to utilize 100% of the molten material in the hearth. Up to half of the molten material remains in the hearth and is not formed into a finished product. Additionally, since the thickness of the solidified product is dependent upon the amount of liquid which is in contact with the drum, and since the hearth rotates about the axis of the drum, the thickness of the final product is initially a function of the height of the pool of liquid in contact with the drum when the hearth is positioned directly to the side of the drum, and then becomes dependent on both the height of the liquid and the angle of the hearth relative to its initial horizontal position. This is due to the changing length of circumferential surface of the drum which the liquid contacts as the hearth rotates around the drum.
Other conventional methods for avoiding problems experienced with the melt overflow process include pivoting the hearth about the point of contact between the rotating drum and the molten material. In many melt overflow processes, a "skull" of solidified material is formed at the lip of the hearth over which the liquid flows. Any rotation of the hearth would cause this skull to contact, and possibly damage, the outer surface of the drum. One way to avoid this contact is to dump a crucible of molten material into a smaller container which is stationary with respect to the axis of the drum. In this case, however, there is a great deal of turbulence created in pouring liquid from one container into the second one.
Therefore, the need arises for an apparatus which extracts most, if not all, of the molten material from the hearth, maintains a constant amount of liquid in contact with the drum, maintains a constant critical distance between the element conveying the liquid to the drum and the outer surface of the drum, and which does not create appreciable turbulence in the liquid during pouring.